the squareneck is useful for at least two reasons. the greater mass in the neck adds sustain to the sound and also allows you to use heavier gauge strings without the neck becoming bowed. most dobroists use strings gauged from .16 on top to as much as.056 on the bottom. it sits on your lap better as well rather than sliding around.
as far as prices go, there are guitars plenty good enough to learn on in the 4-700 dollar range. the really cheap ones (2-300 dollars) that i've seen have such crummy tone that i think it would set back a student's progress because it's almost impossible to produce a good tone on them even when you know how to play!
the most common dobro tuning is open G (GBDGBD bottom to top). that would be the best tuning to start using.
i have a DVD available that is designed for beginning dobro players. if you'd like more info on that mail me at orvillej@comcast.net.